It has long been desired to provide a high definition television picture which approaches the quality of a projected 35 mm photographic film image. See, for example, Image Quality, "A Comparison of Photographic and Television Systems," Otto H. Schade, Jr., RCA Corporation, 1975.
The principal problems in achieving this goal are that any new system must use signals that can be received by conventional television receivers operating according to their respective NTSC, PAL, or SECAM standards, without modification (i.e., fully compatible reception) and that the enhanced signal must be capable of being broadcast to the newly designed receivers without requiring prohibitively large amounts of bandwidth.
Prior art suggestions promising better picture quality but which require greatly increased bandwidth or which are not compatible with existing receivers are disclosed, inter alia, in the SMPTE Journal articles "The Future of High-Definition Television: First Portion of a Report of the SMPTE Study Group on High-Definition Television," by D. G. Fink, Vol. 89, No. 2, February 1980 at pp. 89-94 and Vol. 89, No. 3, March 1980 at pp. 153-161; "High Definition Television System-Signal Standard and Transmission" by T. Fujio, Vol. 89, No. 8, August 1980, pp. 579-584; and "Research and Development on High-Definition Television in Japan" by K. Hayashi, Vol. 90, No. 3, March 1981 at pp. 178-186. In these systems, the lines per frame range from 1023 to 2125 and the required bandwidths range from 20 to 50 megahertz (MHz).
Other techniques labeled as high-definition or high-resolution television systems have advocated some modification of the manner of transmitting the conventional TV signal to provide increased aspect ratio, greater horizontal line resolution or a better filtering of the color subcarrier pattern. U.S. Pat. No. 2,686,831 issued to R. B. Dome on Aug. 17, 1954 proposes to improve picture quality by subdividing the baseband picture signal spectrum into three contiguous bands. The low frequency portion of the spectrum would be transmitted during all picture fields but the medium and "superhigh" frequency portions would be time multiplexed among the odd and even fields, respectively. U.S. Pat. No. 4,296,431 issued to K. F. Holland on Oct. 20, 1981, proposes that the two color axes of the picture signal be inverted at different rates to provide better cancellation of the nonlinear phase shift characteristics of the transmission media. The color subcarrier frequency is chosen as twice the standard (NTSC) frequency to reduce horizontal sweep harmonic interference so that the color subcarrier in adjacent frames may be 180.degree. out of phase.
Another technique for improving the horizontal and vertical definition and for reducing low frequency flicker is disclosed in the article "Concepts For A Compatible HIFI-Television System" by B. Wendland in NTG-Fachber, (Germany), Vol. 7, September, 1980, at pp. 407-416. Double rate image sampling, digital signal processing and offset sampling are used together with image storage at the receiver.
A convenient summary of suggested techniques is contained in recent papers presented at the October 1982 HDTV colloquium of the SMPTE in Ottawa, Canada: "High Definition Television and Its Alternatives," by Ian Childs, B. B. C.; "Signal Processing for Compatible HDTV Systems, First Results," by B. Wendland, Dortmund University, West Germany; "Television Systems For The Future," by T. S. Robson, I. B. A.; and "Compatibility Aspects of HDTV," by Kerns H. Powers, RCA Laboratories.
The Childs paper described split luminance coding for the PAL system to reduce the effects of cross-color. The proposal would use the increased bandwidth available in the satellite channel under the 1977 World Administration Radio Conference (WARC) plan to contain the upper portion of the normal baseband luminance spectrum in a region split away from the band occupied by the chrominance information. The normal high-frequency luminance signals would be conveniently shifted from the normal baseband by using the color subcarrier, i.e., approximately 4.4 MHz in the PAL system. This particular shift was chosen because the color subcarrier is available in both the coder and decoder with accurately known phase and has minimum visability in the decoded signal. The use of only one carrier would also avoid intermodulation in the satellite transponder. However, because the proposed system cannot be received by a standard PAL receiver unless a satellite "front end" were added, the system is not fully compatible with existing conventional (PAL) receivers. Further, the proposed system merely reduces the effects of cross-color without attempting to increase vertical and horizontal resolution.